JavaScript验证 - document.getElementById在Bootstrap模式中不起作用

I am using an onSubmit() function to validate form entries.

The form and validation function work OK,if the form is in a standalone php file or an html file.

But when the form is embedded in a Bootstrap modal the JS validation function throws an error message. [object HTMLInputElement]

I have tried changing document.getElementById('uemail') to document.getElementById('uemail').value as recommended in another answer, but no value has apparently been passed to the function.

If I remove the onSubmit() attribute then the values are passed correctly to the action script on the server.

This must be common usage. What am I doing wrong?

This is an abbreviated section of my code:

<script>
function formCheck() {
    var valid = true;
    //alert('in function');
    var email = document.getElementById('uemail');
    alert('email:' + email);
    var filter = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
    if (!filter.test(email)) {
        alert('Please provide a valid email address');
        email.focus;
        valid = false;
    }
    return valid;
}
</script>


<form id="regform" action="action.php" method="post" onSubmit="return formCheck()">
    Email: <input type="text" name="uemail" id="uemail" size="50" value="" required><span id ="ast">*</span><br>
    <input type="submit" name="Submit" value="Register" />
</form>

EDIT: looks like you can't do document.getElementById, you can do this instead:

function formCheck(formEl) {
    //...
    var email = formEl.getElementsByTagName("input")[0].value;
    // this will get the first input-element in your form

and

onsubmit="return formCheck(this)"

Mistake: The variable email you are testing is a HTMLElement, not a string:

Solution: set email to the value of the HTMLInputElement:

var email = document.getElementById('uemail').value;

also email.focus; is not doing anything, use email.focus();

Demo:

function formCheck() {
  var valid = true;
  //alert('in function');
  var email = document.getElementById('uemail').value;
  alert('email:' + email);
  var filter = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
  if (!filter.test(email)) {
    alert('Please provide a valid email address');
    email.focus;
    valid = false;
  }
  return valid;
}

<form id="regform" action="action.php" method="post" onsubmit="alert(formCheck())">
  Email:
  <input type="text" name="uemail" id="uemail" size="50" value="" required><span id="ast">*</span>
  <br>
  <input type="submit" name="Submit" value="Register" />
</form>

</div>